I. Technical Field
The present invention relates to the measurement of sound velocity. More specifically, the present invention relates to a novel and improved method and apparatus for measuring sound velocity using a time-averaged, phase comparison process to accurately clock the travel time of a pulsed signal transmitted by a transceiver, reflected by a reflector and received at the transceiver.
II. Background Art
Ranging with acoustic signals is the primary method utilized for determining distance in the ocean and positions on the sea floor. The conversion of travel time measurements into distance requires accurate knowledge of the effective sound velocity along the sound propagation path. Currently, the most limiting factor in marine positioning accuracy is the uncertainty in the determination of the ocean's sound velocity field. Present day sea-going sound velocity meters typically have accuracies no better than one part in ten-thousand (1:10.sup.4), which translates into position uncertainties of 100 centimeters in 10 kilometers.
The typical sound velocimeter commonly in use are of the sing-around type The sing-around type velocimeter operates by transmitting a pulsed sinusoidal signal, awaiting detection of the pulse reflection and triggering the transmission of the next pulse with the sequence repeated during the measurement The pulse repetition frequency measured is then related to sound velocity. The resolution of the sing-around type velocimeter is thereby limited by wavelength or frequency. Although the sing-around type sound velocimeter provides accuracy on the level of one part in 10.sup.4, extending the accuracy of sound velocity measurements with this type of sound velocimeter is quite limited. Although the first reflected echo is used to initiate a retransmit of the next pulse, there exist multiple echoes following the first reflected echo which may interfere with the next transmitted pulse. These following reflected echoes interfere with the next transmitted pulse and the first reflected echo of the next transmitted pulse thereby resulting in an apparent phase shift of the first reflected echo of the next transmitted pulse, thereby delaying recognition of the real echo. Furthermore, time delays occur in the detection of the first several cycles of the received echo pulse which delay the transmission of the next pulse.
It is, therefore, an object of the present invention to provide a new and improved method and system for measuring sound velocity.